1. Field of the Invention
The present invention relates generally to an optical transmitter-receiver, and more particularly, to an optical transmitter-receiver in which an optical transmitter and an optical receiver are interconnected such that optical transmission is possible.
2. Description of the Background Art
Optical transmission for transmitting information with light modulated by the information has been expected to be widely used for a future high speed communication network due to low-loss and wideband properties. For example, an optical transmitter-receiver for optically transmitting an electrical signal having a high-frequency (hereinafter referred to as a first optical transmitter-receiver), and an optical transmitter-receiver for optically transmitting a baseband signal (hereinafter referred to as a second optical transmitter-receiver), have been proposed. The two optical transmitter-receivers will be specifically described referring to the drawings.
Description is now made of the first optical transmitter-receiver. In recent years, a wireless service such as a portable telephone or a PHS (Personal Handyphone System) has been rapidly enlarged. Therefore, utilization of still higher frequencies has been examined. A mirco-cell system or a pico-cell system utilizing a millimeter-wave band of approximately 30 GHz to 300 GHz has been examined. In such a cell system, a signal having a high frequency such as a millimeter-wave band is radiated from a lot of base stations connected to a control station, so that a wireless service is provided. The cell system has various advantages. First, the signal having the millimeter-wave band does not easily adversely affect next cells due to a propagation loss in a space. Second, the signal having the millimeter-wave band has a short wavelength, so that an antenna or the like set in the control station or the like is miniaturized. Third, the signal having the millimeter-wave band has a high frequency, so that the transmission capacity can be increased. Consequently, it may be possible to provide a high speed transmission service which is difficult to realize in a conventional wireless service.
In a wireless communication system to which such a cell system is applied, however, a lot of base stations are set throughout a town. Therefore, the base station must be small in size and low in cost. A first optical transmitter-receiver employing a so-called subcarrier optical transmission system which has been tremendously researched and developed in recent years may, in some cases, be applied to the wireless communication system. The subcarrier optical transmission system is described in detail in xe2x80x9cMicrowave and millimeter-wave fiber, optic technologies for subcarrier transmission systemsxe2x80x9d (Hiroyo Ogawa, IEICE Transactions on Communications, Vol. E76-B, No. 9, pp. 1078-1090, September, 1993), for example.
In the subcarrier transmission system, the intensity of a main carrier, which is typically unmodulated light, is modulated by a modulated signal so that an optical signal is obtained. In the modulated signal a subcarrier is modulated by information, which is a voice signal and/or an image signal. The change in the intensity of the optical signal uniquely corresponds to the change in the amplitude, the change in the frequency or the change in the phase of the modulated signal. In the subcarrier optical transmission system, an optical fiber, which is very low in loss, is used. When the modulated signal has a millimeter-wave band, therefore, the modulated signal can be transmitted to a remote location as it is.
FIG. 17 is a block diagram showing the structure of a typical first optical transmitter-receiver. In FIG. 17, the first optical transmitter-receiver comprises a light source 110, an external optical modulating portion 120, an optical fiber 140, an optical/electrical converting portion 150, a frequency converting portion 1710, and a demodulating portion 1720. The light source 110 and the external optical modulating portion 120 constitute an optical transmitter 101, and are set in a base station, while the optical/electrical converting portion 150, the frequency converting portion 1710, and the demodulating portion 1720 constitute an optical receiver 102, and are set in a control station. FIG. 17 shows only signal path in the one direction, that is, the signal path transmitted from the base station to the control station.
In the first optical transmitter-receiver, an electrical signal to be transmitted from the base station to the control station is typically a modulated electrical signal Smod having a millimeter-wave band in which a subcarrier is modulated by a baseband signal such as a voice signal and/or an image signal. The modulated electrical signal Smod is inputted to the external optical modulating portion 120 in the light transmitter 101 through an antenna or an amplifier (not shown) from a portable telephone, a PHS terminal, or the like which is moved outside the base station. The light source 110 oscillates using unmodulated light as a main carrier Mc. The main carrier Mc is also inputted to the external optical modulating portion 120. The external optical modulating portion 120 performs external light-intensity modulation, to modulate the intensity of the inputted main carrier MC on the basis of the change in the amplitude of the inputted modulated electrical signal Smod, thereby obtaining an optical signal OSmod. The optical signal OSmod itself outputted from the external optical modulating portion 120 to the optical fiber 140 is changed into a carrier, and is incident on the optical/electrical converting portion 150 in the optical receiver 102 while the modulated electrical signal Smod is being conveyed through the optical fiber 140 as it is. The optical/electrical converting portion 150 performs optical/electrical conversion, to convert the incident optical signal OSmod into an electrical signal including its intensity modulation component. The frequency converting portion 1710 down-coverts the electrical signal inputted from the optical/electrical converting portion 150 into an electrical signal having an intermediate frequency band. The demodulating portion 1720 demodulates the information of the baseband signal such as the voice signal and/or the image signal on the basis of the electrical signal having the intermediate frequency band inputted from the frequency converting portion 1710.
Description is now made of the second optical transmitter-receiver for merely optically transmitting a baseband signal. FIG. 18 is a block diagram showing the structure of a typical second optical transmitter-receiver. In FIG. 18, the second optical transmitter-receiver comprises a light source driving portion 1810, a light source 110, an optical fiber 140, and an optical/electrical converting portion 150. The light source driving portion 1810 and the light source 110 constitute an optical transmitter 101, while the optical/electrical converting portion 150 constitutes an optical receiver 102. In the second optical transmitter-receiver, it is assumed that a baseband signal SBB to be transmitted from the optical transmitter 101 to the optical receiver 102 is digital information, which is a voice signal and/or an image signal, for example. The baseband signal SBB is inputted to the light source driving portion 1810. The light source driving portion 1810 drives the light source 110, and modulates the intensity of an optical signal outputted from the light source 110 on the basis of the inputted baseband signal SBB (a direct optical modulation system). The optical signal is transmitted through the optical fiber 140, and is then optical/electrical-converted in the optical/electrical converting portion 150, so that the original baseband signal SBB is obtained. Such a light transmission technique is general, and is described in Chapter 2 xe2x80x9cPractice of Optical Communication Systemxe2x80x9d of xe2x80x9cHikari Tsushin Gijyutsu Dokuhon (Optical Transmission Technical Book)xe2x80x9d (edited by Shimada, Ohm Publishing Co., Ltd.) issued in 1980, for example.
However, the optical/electrical converting portion 150 and the frequency converting portion 1710 shown in FIG. 17 must accurately perform optical/electrical conversion and frequency conversion of a signal having a high frequency such as a millimeter-wave band, so that wideband characteristics are required. Otherwise the demodulating portion 1720 would not perform accurate demodulation processing. In the first optical transmitter-receiver, therefore, electrical components corresponding to a high frequency band are interconnected. For this connection, a dedicated connector, waveguide or semirigid cable is used. The waveguide or the semirigid cable is difficult to freely work, so that the first optical transmitter-receiver is difficult to manufacture. It is necessary to use a wave guide, in the case of an attempt to transmit an electrical signal having a high-frequency such as milliwave band with low loss, however, the size of the first transmitter-receiver becomes large, because the size of the waveguide is larger than the size of coaxial cable.
As described in the foregoing, the second optical transmitter-receiver (see FIG. 18) is frequently used for online transmitting the baseband signal SBB, which is digital information, by wire. On the other hand, it is examined whether or not the first optical transmitter-receiver (see FIG. 17) is applied to a wireless communication system. The first and second optical transmitter-receivers are thus examined as separate systems because they differ in their applications. The optical transmitter-receiver for simultaneously optically transmitting both a baseband signal and a high-frequency electrical signal has not been so examined. If a wavelength division multiplexing technique is used, however, such an optical transmitter-receiver can be constructed. That is, the optical signal outputted from the light source 110 shown in FIG. 18 and the optical signal outputted from the external optical modulating portion 120 shown in FIG. 17 are wavelength-division-multiplexed on the transmission side. A signal obtained by the wavelength division multiplexing (WDM) is transmitted through the optical fiber 140, and is separated on the side of optical receiving. Thereafter, signals obtained by the separation are then respectively optical/electrical-converted. Consequently, both the signals are simultaneously obtained on the receiving side. However, the optical transmitter-receiver to which a wavelength division multiplexing technique is applied must separate an optical signal obtained by accurate wavelength division multiplexing on the side of optical receiving. Therefore, a plurality of light sources 110 which differ in oscillation wavelength are required, so that significant cost is required to construct the optical transmitter-receiver.
U.S. Pat. No. 5,596,436 discloses an optical transmitter-receiver to which a subcarrier multiplex optical transmission system is applied, which has apparently similar portions to those in some of optical transmitter-receivers disclosed in the present application. In the optical transmitter-receiver according to the U.S. Patent, however, modulated electrical signals are respectively first produced by modulating subcarriers by baseband signals using mixers. A multiplexed signal is produced by multiplexing the modulated electrical signals by a combiner 40. An external optical modulator 46 modulates unmodulated light from a laser 44 by the multiplexed signal. The optical transmitter according to the U.S. Patent differs in structure from the optical transmitter 101 according to the present invention. That is, a single subcarrier is used in the optical transmitter 101 according to the present invention, while a plurality of subcarriers are used in the optical transmitter according to the U.S. Patent. Consequently, spectrums of optical signals outputted from both the optical transmitters differ from each other. In the optical signal according to the U.S. Patent, a component of a main carrier and a component of each of subcarriers are in close proximity to each other on an optical frequency axis. On the other hand, in an optical signal OS according to the present invention (as described later), a component of a main carrier and components of both sidebands are not in close proximity to each other. Consequently, the optical receiver according to the present invention produces such a significant technical effect that a component of a baseband signal SBB can be taken out more simply and accurately, as compared with that in the U.S. Patent.
Therefore, an object of the present invention is to provide an optical transmitter-receiver capable of optically transmitting a high-frequency electrical signal and being simple in manufacture and small in size.
Another object of the present invention is to provide an optical transmitter-receiver capable of simultaneously optically transmitting both a baseband signal and a high-frequency signal using the same light source.
The first aspect is directed to an optical transmitter-receiver in which an optical transmitter and an optical receiver are interconnected such that an optical transmission is possible, characterized by comprising: a double-modulating portion, to which a subcarrier modulated by an electrical signal to be transmitted is inputted from outside, for double-modulating a main carrier, which is unmodulated light having a predetermined optical frequency, by the inputted subcarrier, to produce and output a double-modulated optical signal, an optical spectrum of the double-modulated optical signal inputted from the double-modulating portion including a component of the main carrier at the predetermined optical frequency and further including components of an upper sideband and a lower sideband at frequencies spaced by the frequency of the subcarrier apart from the predetermined optical frequency, an optical filter portion for selectively passing an optical signal including the component of either one of the upper sideband and the lower sideband in the double-modulated optical signal inputted from the double-modulating portion; and an optical/electrical converting portion for optical/electrical-converting the optical signal inputted from the optical filter portion, to obtain the electrical signal to be transmitted, the optical transmitter comprising at least the local oscillating portion and the double-modulating portion, and the optical receiver comprising at least the optical/electrical converting portion, the optical filter portion being included in either one of the optical transmitter and the optical receiver.
According to the first aspect, the optical/electrical converting portion can directly obtain from the optical signal the electrical signal having a relatively low frequency to be transmitted, thereby eliminating the necessity of an electrical component, which is high in cost and is difficult to process, corresponding to a subcarrier band which is a relatively high frequency as in a conventional optical transmission of a subcarrier. Correspondingly, the optical receiver can be constructed simply and at low cost.
A second aspect is characterized in that in the first aspect, the double-modulating portion comprises a semiconductor laser diode for outputting the main carrier, and at least one external optical modulating portion for amplitude-modulating the main carrier inputted from the semiconductor laser diode by a subcarrier amplitude-modulated by an electrical signal to be transmitted which is inputted from outside using an external optical modulation.
According to the second aspect, the double-modulating portion is constituted by an existing semiconductor laser diode and an existing external optical modulating portion, so that the optical transmitter-receiver is constructed at low cost.
A third aspect is characterized in that in the second aspect, wherein the subcarrier amplitude-modulated by the electrical signal to be transmitted is a signal transmitted from outside, further comprising an antenna portion receiving the signal transmitted for supplying the signal to the double-modulating portion.
According to the third aspect, the optical transmitter-receiver can be easily connected to the wireless transmission system by comprising the antenna portion receiving the signal transmitted from outside.
A fourth aspect is characterized in that in the third aspect, the electrical signal to be transmitted is a multichannel signal frequency-multiplexed, and the electrical modulating portion amplitude-modulates the inputted subcarrier by the multichannel signal, to produce and output the modulated electrical signal.
According to the fourth aspect, the optical transmitter-receiver can optically transmit a lot of information.
A fifth aspect is characterized in that in the third aspect, the electrical signal to be transmitted is digital information, and the electrical modulating portion OOK (on-off keying)-modulates the subcarrier by the digital information.
According to the fifth aspect, the optical transmitter-receiver can transmit information high in quality.
A sixth aspect is directed to an optical transmitter-receiver in which an optical transmitter and an optical receiver are interconnected such that optical transmission is possible, comprising: a double-modulating portion, to which a subcarrier modulated by an electrical signal to be transmitted is inputted from outside, for double-modulating a main carrier, which is unmodulated light having a predetermined optical frequency, by the inputted subcarrier, to produce and output a double-modulated optical signal; an optical spectrum of the double-modulated optical signal inputted from the double-modulating portion including a component of the main carrier at the predetermined optical frequency and further including components of an upper sideband and a lower sideband at frequencies spaced by the frequency of the subcarrier apart from the predetermined optical frequency; an optical filter portion for selectively passing an optical signal including the component of either one of the upper sideband and the lower sideband in the double-modulated optical signal inputted from the double-modulating portion; and an optical branching portion for branching the optical signal inputted from the optical filter portion into a first optical signal and a second optical signal and outputting the first and second optical signals;
a first optical/electrical converting portion for optical/electrical-converting the first optical signal inputted from the optical branching portion, to obtain the electrical signal to be transmitted; a second optical/electrical converting portion for outputting as a detecting signal an electrical signal obtained by optical/electrical-converting the second optical signal inputted from the optical branching portion; and a wavelength control portion for detecting the average values of detected signals inputted from the second optical/electrical converting portion at predetermined time intervals, and controlling the wavelength of the double-modulated optical signal outputted from the double-modulating portion on the basis of the maximum value of the detected average values, the optical transmitter comprising at least the local oscillating portion and the double-modulating portion, the optical receiver comprising at least the first optical/electrical converting portion, and the optical filter portion being included in either one of the optical transmitter and the optical receiver.
According to the sixth aspect, as same as the first aspect, the optical transmitter-receiver can be constructed simply and at low cost, eliminating the necessity of an electrical component, which is high in cost and is difficult to process, corresponding to a subcarrier band which is a relatively high frequency. Further, the optical filter portion can output an optical signal possible a constantly precise demodulation for controlling a wavelength of the double-modulated optical.
A seventh aspect is directed to an optical transmitter-receiver in which an optical transmitter and first and second optical receivers are interconnected such that subcarrier optical transmission is possible, characterized in that the optical transmitter comprises: a local oscillating portion for outputting a subcarrier having a predetermined frequency; a double-modulating portion for double-modulating a main carrier, which is unmodulated light having a predetermined optical frequency, by an electrical signal to be transmitted, which is inputted from outside, and the subcarrier inputted from the local oscillating portion, to produce and output a double-modulated optical signal, a spectrum of the double-modulated optical signal outputted from the double-modulating portion including a component of the main carrier at the predetermined optical frequency and further including components of an upper sideband and a lower sideband at frequencies spaced by the frequency of the subcarrier apart from the predetermined optical frequency, and an optical portion for dividing the double-modulated optical signal inputted from the double-modulating portion into a first optical signal including the component of either one of the upper sideband and the lower sideband and a second optical signal including the component of the main carrier and the component of the other one of the upper sideband and the lower sideband, to output the first optical signal and the second optical signal, the first optical receiver optical/electrical-converts the first optical signal transmitted from the optical transmitter, to obtain the electrical signal to be transmitted, and the second optical receiver optical/electrical converts the second optical signal transmitted from the optical transmitter, to output the subcarrier that is modulated by the electrical signal to be transmitted.
The first optical signal includes the component of one of the sidebands included in the double-modulated optical signal obtained by the double modulation, and is optical/electrical-converted by the first optical/electrical converting portion, to be converted into an electrical signal to be transmitted. Further, the second optical signal includes the components of the other sideband and the main carrier in the double-modulated optical signal, and is optical/electrical-converted by the second optical/electrical converting portion, to be converted into a signal in which the subcarrier is modulated by the electrical signal to be transmitted. According to the seventh aspect, both the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted can be simultaneously obtained on the receiving side. Further, as apparent by referring to the foregoing, both the signals can be transmitted by a signal wave of unmodulated light, so that the optical transmitter-receiver can be constructed at low cast according to the seventh aspect without requiring a plurality of light sources as in a wavelength division multiplexing technique.
An eighth aspect is characterized in that in the seventh aspect, the double-modulating portion comprises: an electrical modulating portion for amplitude-modulating the subcarrier inputted from the local oscillating portion by the electrical signal to be transmitted, which is inputted from outside, to produce and output a modulated electrical signal; a light source for outputting the main carrier, which is unmodulated light having a predetermined optical frequency; and an external optical modulating portion for amplitude-modulating the main carrier inputted from the light source by the modulated electrical signal inputted from the electrical modulating portion, to produce the double-modulated optical signal.
According to the eighth aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost.
A ninth aspect is characterized in that in the eighth aspect, the electrical signal to be transmitted is digital information, and the electrical modulating portion OOK (on-off keying)-modulates the subcarrier by the digital information.
According to the ninth aspect, the optical transmitter-receiver can transmit information high in quality.
A tenth aspect is characterized in that in the seventh aspect, the double-modulating portion comprises: a light source for outputting the main carrier, which is unmodulated light having a predetermined optical frequency; a first external optical modulating portion for amplitude-modulating the main carrier inputted from the light source by the subcarrier inputted from the local oscillating portion, to produce and output a modulated optical signal; and a second external optical modulating portion for amplitude-modulating the modulated optical signal inputted from the first external optical modulating portion by the electrical signal to be transmitted, which is inputted from outside, to produce the double-modulated optical signal.
According to the tenth aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost.
An eleventh aspect is characterized in that in the seventh aspect, the double-modulating portion comprises: a light source for outputting the main carrier which is unmodulated light having a predetermined optical frequency; a first external optical modulating portion for amplitude-modulating the main carrier inputted from the light source by the electrical signal to be transmitted, which is inputted from outside, to produce and output a modulated optical signal; and a second external optical modulating portion for amplitude-modulating the modulated optical signal inputted from the first external optical modulating portion by the subcarrier inputted from the local oscillating portion, to produce the double-modulated optical signal.
According to the eleventh aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost.
A twelfth aspect is characterized in that in the seventh aspect, the optical filter portion comprises an optical circular portion for outputting the double-modulated optical signal inputted from the double-modulating portion as it is, and an optical fiber grating portion for reflecting the component of either one of the upper sideband and the lower sideband in the double-modulated optical signal inputted from the optical circulator portion, to produce the first optical signal and output the produced first optical signal to the optical circulator portion, and passing the component of the main carrier and the component of the other one of the upper sideband and the lower sideband, to produce and output the second optical signal to the second optical receiver, the optical circulator portion further outputting the first optical signal inputted from the optical fiber grating portion as it is to the first optical receiver.
In the twelfth aspect, the optical filter portion is constituted by the optical circulator and the optical fiber grating which are optical components, so that the optical transmitter-receiver is constructed simply and at low cost.
A thirteenth aspect is characterized in that in the seventh aspect, the second optical receiver comprises an antenna portion for radiating to a space the subcarrier that is modulated by the electrical signal to be transmitted which is obtained by the optical/electrical conversion.
The subcarrier modulated by the electrical signal to be transmitted is a signal suitable for wireless transmission. According to the thirteenth aspect, the second optical receiver comprises the antenna portion for radiating the subcarrier to a space, so that the optical transmitter-receiver is easily connected to a wireless transmission system.
A fourteenth aspect is characterized in that in the seventh aspect, the electrical signal to be transmitted is an electrical signal to be transmitted which is converted analog information into digital information.
According to the fourteenth aspect, the optical transmitter-receiver can transmit information high in quality.
A fifteenth aspect is characterized in that in the seventh aspect, the electrical signal to be transmitted is a carrier modulated by analog information and digital information, the frequency of the carrier is an intermediate frequency lower than that of the subcarrier outputted from the local oscillating portion.
When the electrical signal to be transmitted is the above-mentioned electrical signal, the carrier having the intermediate frequency modulated by the analog information or the like and the signal in which the subcarrier is modulated by the carrier having the intermediate frequency are obtained on the receiving side of the optical transmitter-receiver according to the fifteenth aspect. Consequently, the optical transmitter-receiver can perform optical transmission which does not depend on a modulation form.
A sixeenth aspect is characterized in that in the seventh aspect, the electrical signal to be transmitted is obtained by multiplexing a plurality of electrical signals that have the intermediate frequency and are modulated by analog information or digital information using a predetermined multiplexing technique, respectively.
A seventeenth aspect is characterized in that in the sixteenth aspect, the predetermined multiplexing technique is a frequency division multiplexing access, a time division multiplexing access or a code division multiplexing access.
According to the sixteenth and seventeenth aspects, the optical transmitter-receiver can multiplex a lot of information and optically transmit information obtained by the multiplexing.
An eighteenth aspect is directed to an optical transmitter-receiver in which an optical transmitter and first and second optical receivers are interconnected such that subcarrier optical information is possible, characterized in that the optical transmitter comprises: a local oscillating portion for outputting a subcarrier having a predetermined frequency; a double-modulating portion for double-modulating a main carrier, which is unmodulated light having a predetermined optical frequency, by an electrical signal to be transmitted, which is inputted from outside, and by the subcarrier inputted from the local oscillating portion, to produce and output a double-modulated optical signal; and an optical branching portion for branching the double-modulated optical signal inputted from the double-modulating portion and outputting double-modulated optical signals obtained by the branching, the first optical receiver comprises a low-pass filter portion for passing a component included in a low frequency band of an electrical signal obtained by optical/electrical-converting the double-modulated optical signal transmitted from the optical transmitter, to output the electrical signal to be transmitted, and the second optical receiver comprises a high-pass filter portion for passing a component included in a high frequency band of an electrical signal obtained by optical/electrical-converting the double-modulated optical signal transmitted from the optical transmitter, to output the subcarrier that is modulated by the electrical signal to be transmitted.
On the receiving side in the eighteenth aspect, as same as the seventh aspect, the low-pas filter portion and the high-pass filter portion respectively pass a low frequency band part and a high frequency band part of the electrical signal obtained by optical/electrical-converting the double-modulated optical signal. Therefore, signals obtained upon respectively modulating the subcarrier by an electrical signal to be transmitted, which is included in the relatively low frequency band, and an electrical signal to be transmitted, which is included in the relatively high frequency band, can be simultaneously obtained. Further, the optical transmitter-receiver can be constructed at low cost.
A nineteenth aspect is characterized in that in the eighteenth aspect, the double-modulating portion comprises: an electrical modulating portion for amplitude-modulating the subcarrier outputted from the local oscillating portion by the electrical signal to be transmitted, which is inputted from outside, to produce and output a modulated electrical signal; a light source for outputting the main carrier, which is unmodulated light having a predetermined optical frequency; and an external optical modulating portion for amplitude-modulating the main carrier outputted from the light source by the modulated electrical signal inputted from the electrical modulating portion, to produce the double-modulated optical signal.
According to the nineteenth aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost.
A twentieth aspect is characterized in that in the nineteenth aspect, the electrical signal to be transmitted is digital information, and the electrical modulating portion OOK (on-off keying)-modulates the subcarrier by the digital information.
According to the twentieth aspect, the optical transmitter-receiver can transmit information high in quality.
A twenty-first aspect is characterized in that in the eighteenth aspect, the double-modulating portion comprises: a light source for outputting the main carrier which is unmodulated light having a predetermined optical frequency; a first external optical modulating portion for amplitude-modulating the main carrier inputted from the light source by the subcarrier inputted from the local oscillating portion, to produce and output a modulated optical signal; and a second external optical modulating portion for amplitude-modulating the modulated optical signal inputted from the first external optical modulating portion by the electrical signal to be transmitted which is inputted from outside, to produce the double-modulated optical signal.
According to the twenty-first aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost. p A twenty-second aspect is characterized in that in the eighteenth aspect, the double-modulating portion comprises: a light source for outputting the main carrier, which is unmodulated light having a predetermined optical frequency; a first external optical modulating portion for amplitude-modulating the main carrier inputted from the light source by the electrical signal to be transmitted, which is inputted from outside, to produce and output a modulated optical signal; and a second external optical modulating portion for amplitude-modulating the modulated optical signal inputted from the first external optical modulating portion by the subcarrier inputted from the local oscillating portion, to produce the double-modulated optical signal.
According to the twenty-second aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost.
A twenty-third aspect is characterized in that in the eighteenth aspect, an antenna portion for radiating to a space is set in a back end against the high-pass filter portion. The antenna portion radiates the subcarrier that is modulated by the electrical signal to be transmitted, which is outputted from the high-pass filter portion.
According to the twenty-third aspect, the optical transmitter-receiver is simply connected to a wireless transmission system, as in the thirteenth aspect.
A twenty-fourth aspect, is characterized in that in the eighteenth aspect, the electrical signal to be transmitted is a carrier modulated by analog information or digital information, the frequency of the carrier is an intermediate frequency lower than that of the subcarrier outputted from the local oscillating portion.
According to the twenty-fourth aspect, when the electrical signal to be transmitted is the above-mentioned electrical signal, the carrier having the intermediate frequency modulated by the analog information or the like and the signal in which the subcarrier is modulated by the carrier having the intermediate frequency are obtained on the side of optical receiving. Consequently, the optical transmitter-receiver can perform optical transmission which does not depend on a modulation form.
A twenty-fifth aspect is characterized in that the eighteenth aspect, the double-modulating portion modulates the main carrier by the subcarrier inputted from the local oscillating portion using a single sideband amplitude modulation system.
In the twenty-fifth aspect, the double-modulated optical signal is not easily affected by wavelength dispersion in an optical fiber serving as an optical transmission line by applying a single sideband amplitude modulation system, so that the transmission distance increases.
A twenty-sixth aspect is directed to an optical transmitter-receiver in which an optical transmitter and an optical receiver are interconnected such that subcarrier optical transmission is possible, characterized in that the optical transmitter comprises: a local oscillating portion for outputting a subcarrier having a predetermined frequency; and a double-modulating portion for double-modulating a main carrier, which is unmodulated light having a predetermined optical frequency, by an electrical signal to be transmitted, which is inputted from outside, and by the subcarrier inputted from the local oscillating portion, to produce and output a double-modulated optical signal, and the optical receiver comprises an optical/electrical converting portion for optical/electrical-converting the double-modulated optical signal transmitted from the optical transmitter, to output an electrical signal; a distributing portion for distributing the electrical signal inputted from the optical/electrical converting portion into at least two electrical signals; a low-pass filter portion for passing a component included in a low frequency band of the electrical signal obtained by the distribution, to output the electrical signal to be transmitted; and a high-pass filter portion for passing a component included in a high frequency band of the electrical signal obtained by the distribution, to output the subcarrier that is modulated by the electrical signal to be transmitted.
On the receiving side in the twenty-sixth aspect, the low-pass filter portion and the high-pass filter portion respectively pass a low frequency band part and a high frequency band part of the electrical signal obtained by optical/electrical-converting the double-modulated optical signal, as in the seventh aspect. Therefore, signals in which a subcarrier is modulated by an electrical signal to be transmitted, which is included in the relatively low frequency band, and an electrical signal to be transmitted, which is included in the relatively high frequency band, can be simultaneously obtained. Further, the optical transmitter-receiver can be constructed at low cost.
A twenty-seventh aspect is characterized in that in the twenty-sixth aspect, the double-modulating portion comprises: an electrical modulating portion for amplitude-modulating the subcarrier inputted from the local oscillating portion by the electrical signal to be transmitted, which is inputted from outside, to produce and output a modulated electrical signal; a light source for outputting the main carrier, which is unmodulated light having a predetermined optical frequency; and an external optical modulating portion for amplitude-modulating the main carrier inputted from the light source by the modulated electrical signal inputted from the electrical modulating portion, to produce the double-modulated optical signal.
According to the twenty-seventh aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost.
A twenty-eighth aspect is characterized in that in the twenty-seventh aspect, the electrical signal to be transmitted is digital information, and the electrical modulating portion OOK (on-off keying)-modulates the subcarrier by the digital information.
According to the twenty-eighth aspect, the optical transmitter-receiver can transmit information high in quality.
A twenty-ninth aspect is characterized in that in the twenty-sixth aspect, the double-modulating portion comprises: a light source for outputting the main carrier, which is unmodulated light having a predetermined optical frequency; a first external optical modulating portion for amplitude-modulating the main carrier inputted from the light source by the subcarrier inputted from the local oscillating portion, to produce and output a modulated optical signal; and a second external optical modulating portion for amplitude-modulating the modulated optical signal inputted from the first external optical modulating portion by the electrical signal to be transmitted, which is inputted from outside, to produce the double-modulated optical signal.
According to the twenty-ninth aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost.
A thirtieth aspect is characterized in that in the twenty-sixth aspect, the double-modulating portion comprises: a light source for outputting the main carrier, which is unmodulated light having a predetermined optical frequency; a first external optical modulating portion for amplitude-modulating the main carrier inputted from the light source by the electrical signal to be transmitted, which is inputted from outside, to produce and output a modulated optical signal; and a second external optical modulating portion for amplitude-modulating the modulated optical signal inputted from the first external optical modulating portion by the subcarrier inputted from the local oscillating portion, to produce the double-modulated optical signal.
According to the thirtieth aspect, the optical transmitter uses the same light source to simultaneously transmit the electrical signal to be transmitted and the signal in which the subcarrier is modulated by the electrical signal to be transmitted toward the receiving side. Consequently, the optical transmitter-receiver is constructed at low cost.
A thirty-first aspect is characterized in that in the twenty-sixth aspect, an antenna portion for radiating to a space is set in a back end against the high-pass filter portion. The antenna portion radiates the subcarrier that is modulated by the electrical signal to be transmitted, which is outputted from the high-pass filter portion.
According to the thirty-first aspect, the optical transmitter-receiver is simply connected to a wireless transmission system, as in the thirteenth aspect.
A thirty-second aspect is characterized in that in the twenty-sixth aspect, the electrical signal to be transmitted is a carrier modulated by analog information or digital information, the frequency of the carrier is an intermediate frequency lower than that of the subcarrier outputted from the local oscillating portion.
According to the thirty-second aspect, the optical transmitter-receiver can perform optical transmission which does not depend on a modulation form, as in the fifteenth aspect.
A thirty-third aspect is characterized in that in the twenty-sixth aspect, the double-modulating portion modulates the main carrier by the subcarrier inputted from the local oscillating portion using a single sideband amplitude modulation system.
In the thirty-third aspect, the double-modulated optical signal is not easily affected by wavelength dispersion in an optical fiber serving as an optical transmission line, so that the transmission distance increases, as in the twenty-fifth aspect.
A thirty-fourth aspect is directed to an optical transmitter-receiver in which an optical transmitter and first and second optical receivers are interconnected such that subcarrier optical transmission is possible, characterized in that the optical transmitter comprises: a local oscillating portion for outputting a subcarrier having a predetermined frequency; a mode locked light source which is mode-locked on the basis of the subcarrier inputted from the local oscillating portion and oscillating with spacing between optical frequencies related to the subcarrier, to produce and output a mode-locked optical signal; an external optical modulating portion for amplitude-modulating the mode-locked optical signal inputted from the mode locked light source by an electrical signal to be transmitted, which is inputted from outside, to produce and output a double-modulated optical signal; and an optical branching portion for branching the double-modulated optical signal inputted from the external optical modulating portion and outputting double-modulated optical signals obtained by the branching, the first optical receiver comprises a low-pass filter portion for passing a component included in a low frequency band of an electrical signal obtained by optical/electrical-converting the double-modulated optical signal transmitted from the optical transmitter, to output the electrical signal to be transmitted, and the second optical receiver comprises a high-pass filter portion for passing a component included in a high frequency band of an electrical signal obtained by optical/electrical-converting the double-modulated optical signal transmitted from the optical transmitter, to output the subcarrier that is modulated by the electrical signal to be transmitted.
On the receiving side in the thirty-fourth aspect, the low-pass filter portion and the high-pass filter portion respectively pass a low frequency band part and a high frequency band part of the electrical signal obtained by optical/electrical-converting the double-modulated optical signal, as in the seventh aspect. Therefore, signals in which the subcarrier is modulated by an electrical signal to be transmitted, which is included in the relatively low frequency band, and an electrical signal to be transmitted, which is included in the relatively high frequency band, can be simultaneously obtained. Further, the optical transmitter-receiver can be constructed at low cost.
A thirty-fifth aspect is characterized in that in the thirty-fourth aspect, an antenna portion for radiating to a space is set in a back end against the high-pass filter portion. The antenna portion radiates the subcarrier that is modulated by the electrical signal to be transmitted, which is outputted from the high-pass filter portion.
According to the thirty-fifth aspect, the optical transmitter-receiver is simply connected to a wireless transmission system, as in the thirteenth aspect.
A thirty-sixth aspect is characterized in that in the thirty-fourth aspect, the electrical signal to be transmitted is a carrier modulated by analog information or digital information, the frequency of the carrier is an intermediate frequency lower than that of the subcarrier outputted from the local oscillating portion.
According to the thirty-sixth aspect, the optical transmitter-receiver can perform optical transmission which does not depend on a modulation form, as in the fifteenth aspect.
A thirty-seventh aspect is directed to an optical transmitter-receiver in which an optical transmitter and an optical receiver are interconnected such that subcarrier optical transmission is possible, characterized in that the optical transmitter comprises: a local oscillating portion for outputting a subcarrier having a predetermined frequency; a mode locked light source which is mode-locked on the basis of a subcarrier inputted from the local oscillating portion and oscillating with spacing between optical frequencies related to the subcarrier, to produce and output a mode-locked optical signal, and an external optical modulating portion for amplitude-modulating the mode-locked optical signal inputted from the mode locked light source by the electrical signal to be transmitted, which is inputted from outside, to produce and output a double-modulated optical signal, and the optical receiver comprises: an optical/electrical converting portion for optical/electrical-converting the double-modulated optical signal transmitted from the optical transmitter, to output an electrical signal; a distributing portion for distributing the electrical signal inputted from the optical/electrical converting portion into at least two electrical signals; a low-pass filter portion for passing a component included in a low frequency band of the electrical signal obtained by the distribution, to output the electrical signal to be transmitted, and a high-pass filter portion for passing a component included in a high frequency band of the electrical signal obtained by the distribution, to output the subcarrier that is modulated by the electrical signal to be transmitted.
On the receiving side in the thirty-seventh aspect, the low-pass filter portion and the high-pass filter portion respectively pass a low frequency band part and a high frequency band part of the electrical signal obtained by optical/electrical-converting the double-modulated optical signal, as in the seventh aspect. Therefore, signals obtained by modulating the subcarrier by an electrical signal to be transmitted, which is included in the relatively low frequency band, and an electrical signal to be transmitted, which is included in the relatively high frequency band, can be simultaneously obtained. Further, the optical transmitter-receiver can be constructed at low cost.
A thirty-eighth aspect is characterized in that in the thirty-seventh aspect, an antenna portion for radiating to a space is set in a back end against the high-pass filter portion. The antenna portion radiates the subcarrier that is modulated by the electrical signal to be transmitted which is outputted from the high-pass filter portion.
According to the thirty-eighth aspect, the optical transmitter-receiver is simply connected to a wireless transmission system, as in the thirteenth aspect.
A thirty-ninth aspect is characterized in that in the thirty-seventh aspect, the electrical signal to be transmitted is a carrier modulated by analog information or digital information, the frequency of the carrier is an intermediate frequency lower than that of the subcarrier outputted from the local oscillating portion.
According to the thirty-ninth aspect, the optical transmitter-receiver can perform optical transmission which does not depend on a modulation form, as in the fifteenth aspect.
A fortieth aspect is directed to an optical transmitter-receiver in which an optical transmitter and first and second optical receivers are interconnected such that optical transmission is possible, wherein the optical transmitter comprises: a first light source for outputting first unmodulated light having a first optical frequency; an external optical modulating portion for amplitude-modulating the first unmodulated light inputted from the first light source by an electrical signal to be transmitted, which is inputted from outside, to produce and output a modulated optical signal; a second light source for outputting second unmodulated light having a second optical frequency, which differs from the first optical frequency by a predetermined optical frequency; an optical multiplexing portion for multiplexing the modulated optical signal inputted from the external optical modulating portion and the second unmodulated light inputted from the second light source such that polarization of the modulated optical signal and the second unmodulated light coincide with each other, to produce and output an optical signal; and an optical branching portion for branching the optical signal inputted from the optical multiplexing portion and outputting optical signals obtained by the branching, the first optical receiver comprises a low-pass filter portion for passing a component included in a low frequency band of an electrical signal obtained by optical/electrical-converting the optical signal transmitted from the optical transmitter, to output the electrical signal to be transmitted, and the second optical receiver comprises a high-pass filter portion for passing a component included in a high frequency band of an electrical signal obtained by optical/electrical converting the optical signal transmitted from the optical transmitter, to output the subcarrier that is modulated by the electrical signal to be transmitted.
According to the fortieth aspect, the first unmodulated light is amplitude-modulated by the electrical signal to be transmitted, to produce the modulated optical signal. The modulated optical signal and the second unmodulated light are multiplexed, to produce the optical signal. Although optical/electrical conversion must be made twice in the seventh aspect, for example, the optical transmitter in the fortieth aspect performs optical/electrical conversion only once. By thus reducing the number of times of optical/electrical conversion, low-loss optical transmission can be realized. Further, in the optical transmitter in the fortieth aspect, no electrical component for amplitude-modulating the subcarrier by the electrical signal to be transmitted is required. That is, according to the fortieth aspect, the necessity of an electrical component, which is high in cost and is difficult to process, corresponding to a subcarrier band which is a relatively high frequency is eliminated. Correspondingly, the optical receiver can be constructed simply and at low cost.
A forty-first aspect is characterized in that in the fortieth aspect, an antenna portion for radiating to a space is set in a back end against the high-pass filter portion. The antenna portion radiates the subcarrier that is modulated by the electrical signal to be transmitted, which is outputted from the high-pass filter portion.
According to the forty-first aspect, the optical transmitter-receiver is simply connected to a wireless transmission system, as in the thirteenth aspect.
A forty-second aspect is characterized in that in the fortieth aspect, the electrical signal to be transmitted is a carrier modulated by analog information or digital information, the frequency of the carrier is an intermediate frequency lower than that of the subcarrier outputted from the local oscillating portion.
According to the forty-second aspect, the optical transmitter-receiver can perform optical transmission which does not depend on a modulation form, as in the fifteenth aspect.
A forty-third aspect is directed to an optical transmitter-receiver in which an optical transmitter and an optical receiver are interconnected such that optical transmission is possible, wherein the optical transmitter comprises: a first light source for outputting first unmodulated light having a first optical frequency, an external optical modulating portion for amplitude-modulating the first unmodulated light inputted from the first light source by an electrical signal to be transmitted, which is inputted from outside, to produce and output a modulated optical signal; a second light source for outputting second unmodulated light having a second optical frequency, which differs from the first optical frequency by a predetermined optical frequency; an optical multiplexing portion for multiplexing the modulated optical signal inputted from the external optical modulating portion and the second unmodulated light inputted from the second light source such that polarization of the modulated optical signal and the second unmodulated light coincide with each other, to produce and output an optical signal; and an optical branching portion for branching the optical signal inputted from the optical multiplexing portion and outputting optical signals branched by the branching portion, and the optical receiver comprises: an optical/electrical converting portion for optical/electrical-converting the optical signal transmitted from the optical transmitter, to output an electrical signal; a distributing portion for distributing the electrical signal inputted from the optical/electrical converting portion into at least two electrical signals; a low-pass filter portion for passing a component included in a low frequency band of the electrical signal obtained by the distribution, to output the electrical signal to be transmitted; and a high-pass filter portion for passing a component included in a high frequency band of the electrical signal obtained by the distribution, to output the subcarrier that is modulated by the electrical signal to be transmitted.
According to the forty-third aspect, it is possible to realize low-loss optical transmission as well as to construct the optical transmitter-receiver simply and at low cost.
A forty-fourth aspect is characterized in that in the forty-third aspect, an antenna portion for radiating to a space is set in a back end against the high-pass filter portion. The antenna portion radiates the subcarrier that is modulated by the electrical signal to be transmitted, which is outputted from the high-pass filter portion.
According to the forty-fourth aspect, the optical transmitter-receiver is simply connected to a wireless transmission system, as in the thirteenth aspect.
A forty-fifth aspect is characterized in that in the forty-third aspect, the electrical signal to be transmitted is a carrier modulated by analog information or digital information, the frequency of the carrier is an intermediate frequency lower than that of the subcarrier outputted from the local oscillating portion.
According to the forty-fifth aspect, the optical transmitter-receiver can perform optical transmission which does not depend on a modulation form, as in the fifteenth aspect.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.